Someone who is at home watching a sport event (such as football etc.) on television at home and thinks that they know how to play the sport better than those that are actually competing. This leads to unnecessary yelling of comments towards the screen, which the athletes can not hear anyway.
A couch coach can be any sort of television sport spectator, but is often someone that lacks any real sporting ability.
Derived from the television environment (couch) and the constant yelling similar to that of a coach. Also just a fun spin on the classic "couch potato".